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<head><title>HTTP/1.1: Introduction</title></head>
<body><address>part of <a rev='Section' href='rfc2616.html'>Hypertext Transfer Protocol -- HTTP/1.1</a><br />
RFC 2616 Fielding, et al.</address>
<h2><a id='sec1'>1</a> Introduction</h2>
<h3><a id='sec1.1'>1.1</a> Purpose</h3>
<p>
   The Hypertext Transfer Protocol (HTTP) is an application-level
   protocol for distributed, collaborative, hypermedia information
   systems. HTTP has been in use by the World-Wide Web global
   information initiative since 1990. The first version of HTTP,
   referred to as HTTP/0.9, was a simple protocol for raw data transfer
   across the Internet. HTTP/1.0, as defined by RFC 1945 <a rel='bibref' href='rfc2616-sec17.html#bib6'>[6]</a>, improved
   the protocol by allowing messages to be in the format of MIME-like
   messages, containing metainformation about the data transferred and
   modifiers on the request/response semantics. However, HTTP/1.0 does
   not sufficiently take into consideration the effects of hierarchical
   proxies, caching, the need for persistent connections, or virtual
   hosts. In addition, the proliferation of incompletely-implemented
   applications calling themselves "HTTP/1.0" has necessitated a
   protocol version change in order for two communicating applications
   to determine each other's true capabilities.
</p>
<p>
   This specification defines the protocol referred to as "HTTP/1.1".
   This protocol includes more stringent requirements than HTTP/1.0 in
   order to ensure reliable implementation of its features.
</p>
<p>
   Practical information systems require more functionality than simple
   retrieval, including search, front-end update, and annotation. HTTP
   allows an open-ended set of methods and headers that indicate the
   purpose of a request <a rel='bibref' href='rfc2616-sec17.html#bib47'>[47]</a>. It builds on the discipline of reference
   provided by the Uniform Resource Identifier (URI) <a rel='bibref' href='rfc2616-sec17.html#bib3'>[3]</a>, as a location
   (URL) <a rel='bibref' href='rfc2616-sec17.html#bib4'>[4]</a> or name (URN) <a rel='bibref' href='rfc2616-sec17.html#bib20'>[20]</a>, for indicating the resource to which a
</p>
<p>
   method is to be applied. Messages are passed in a format similar to
   that used by Internet mail [9] as defined by the Multipurpose
   Internet Mail Extensions (MIME) <a rel='bibref' href='rfc2616-sec17.html#bib7'>[7]</a>.
</p>
<p>
   HTTP is also used as a generic protocol for communication between
   user agents and proxies/gateways to other Internet systems, including
   those supported by the SMTP <a rel='bibref' href='rfc2616-sec17.html#bib16'>[16]</a>, NNTP <a rel='bibref' href='rfc2616-sec17.html#bib13'>[13]</a>, FTP <a rel='bibref' href='rfc2616-sec17.html#bib18'>[18]</a>, Gopher <a rel='bibref' href='rfc2616-sec17.html#bib2'>[2]</a>,
   and WAIS <a rel='bibref' href='rfc2616-sec17.html#bib10'>[10]</a> protocols. In this way, HTTP allows basic hypermedia
   access to resources available from diverse applications.
</p>
<h3><a id='sec1.2'>1.2</a> Requirements</h3>
<p>
   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 <a rel='bibref' href='rfc2616-sec17.html#bib34'>[34]</a>.
</p>
<p>
   An implementation is not compliant if it fails to satisfy one or more
   of the MUST or REQUIRED level requirements for the protocols it
   implements. An implementation that satisfies all the MUST or REQUIRED
   level and all the SHOULD level requirements for its protocols is said
   to be "unconditionally compliant"; one that satisfies all the MUST
   level requirements but not all the SHOULD level requirements for its
   protocols is said to be "conditionally compliant."
</p>
<h3><a id='sec1.3'>1.3</a> Terminology</h3>
<p>
   This specification uses a number of terms to refer to the roles
   played by participants in, and objects of, the HTTP communication.
</p>
<dl>
 <dt>   connection
</dt> <dd>      A transport layer virtual circuit established between two programs
      for the purpose of communication.
</dd>
 <dt>   message
</dt> <dd>      The basic unit of HTTP communication, consisting of a structured
      sequence of octets matching the syntax defined in section 4 and
      transmitted via the connection.
</dd>
 <dt>   request
</dt> <dd>      An HTTP request message, as defined in section 5.
</dd>
 <dt>   response
</dt> <dd>      An HTTP response message, as defined in section 6.
</dd>
 <dt>   resource
</dt> <dd>      A network data object or service that can be identified by a URI,
      as defined in section <a rel='xref' href='rfc2616-sec3.html#sec3.2'>3.2</a>. Resources may be available in multiple
      representations (e.g. multiple languages, data formats, size, and
      resolutions) or vary in other ways.
</dd>
 <dt>   entity
</dt> <dd>      The information transferred as the payload of a request or
      response. An entity consists of metainformation in the form of
      entity-header fields and content in the form of an entity-body, as
      described in section 7.
</dd>
 <dt>   representation
</dt> <dd>      An entity included with a response that is subject to content
      negotiation, as described in section 12. There may exist multiple
      representations associated with a particular response status.
</dd>
 <dt>   content negotiation
</dt> <dd>      The mechanism for selecting the appropriate representation when
      servicing a request, as described in section 12. The
      representation of entities in any response can be negotiated
      (including error responses).
</dd>
 <dt>   variant
</dt> <dd>      A resource may have one, or more than one, representation(s)
      associated with it at any given instant. Each of these
      representations is termed a `varriant'.  Use of the term `variant'
      does not necessarily imply that the resource is subject to content
      negotiation.
</dd>
 <dt>   client
</dt> <dd>      A program that establishes connections for the purpose of sending
      requests.
</dd>
 <dt>   user agent
</dt> <dd>      The client which initiates a request. These are often browsers,
      editors, spiders (web-traversing robots), or other end user tools.
</dd>
 <dt>   server
</dt> <dd>      An application program that accepts connections in order to
      service requests by sending back responses. Any given program may
      be capable of being both a client and a server; our use of these
      terms refers only to the role being performed by the program for a
      particular connection, rather than to the program's capabilities
      in general. Likewise, any server may act as an origin server,
      proxy, gateway, or tunnel, switching behavior based on the nature
      of each request.
</dd>
 <dt>   origin server
</dt> <dd>      The server on which a given resource resides or is to be created.
</dd>
 <dt>   proxy
</dt> <dd>      An intermediary program which acts as both a server and a client
      for the purpose of making requests on behalf of other clients.
      Requests are serviced internally or by passing them on, with
      possible translation, to other servers. A proxy MUST implement
      both the client and server requirements of this specification. A
      "transparent proxy" is a proxy that does not modify the request or
      response beyond what is required for proxy authentication and
      identification. A "non-transparent proxy" is a proxy that modifies
      the request or response in order to provide some added service to
      the user agent, such as group annotation services, media type
      transformation, protocol reduction, or anonymity filtering. Except
      where either transparent or non-transparent behavior is explicitly
      stated, the HTTP proxy requirements apply to both types of
      proxies.
</dd>
 <dt>   gateway
</dt> <dd>      A server which acts as an intermediary for some other server.
      Unlike a proxy, a gateway receives requests as if it were the
      origin server for the requested resource; the requesting client
      may not be aware that it is communicating with a gateway.
</dd>
 <dt>   tunnel
</dt> <dd>      An intermediary program which is acting as a blind relay between
      two connections. Once active, a tunnel is not considered a party
      to the HTTP communication, though the tunnel may have been
      initiated by an HTTP request. The tunnel ceases to exist when both
      ends of the relayed connections are closed.
</dd>
 <dt>   cache
</dt> <dd>      A program's local store of response messages and the subsystem
      that controls its message storage, retrieval, and deletion. A
      cache stores cacheable responses in order to reduce the response
      time and network bandwidth consumption on future, equivalent
      requests. Any client or server may include a cache, though a cache
      cannot be used by a server that is acting as a tunnel.
</dd>
 <dt>   cacheable
</dt> <dd>      A response is cacheable if a cache is allowed to store a copy of
      the response message for use in answering subsequent requests. The
      rules for determining the cacheability of HTTP responses are
      defined in section 13. Even if a resource is cacheable, there may
      be additional constraints on whether a cache can use the cached
      copy for a particular request.
</dd>
 <dt>   first-hand
</dt> <dd>      A response is first-hand if it comes directly and without
      unnecessary delay from the origin server, perhaps via one or more
      proxies. A response is also first-hand if its validity has just
      been checked directly with the origin server.
</dd>
 <dt>   explicit expiration time
</dt> <dd>      The time at which the origin server intends that an entity should
      no longer be returned by a cache without further validation.
</dd>
 <dt>   heuristic expiration time
</dt> <dd>      An expiration time assigned by a cache when no explicit expiration
      time is available.
</dd>
 <dt>   age
</dt> <dd>      The age of a response is the time since it was sent by, or
      successfully validated with, the origin server.
</dd>
 <dt>   freshness lifetime
</dt> <dd>      The length of time between the generation of a response and its
      expiration time.
</dd>
 <dt>   fresh
</dt> <dd>      A response is fresh if its age has not yet exceeded its freshness
      lifetime.
</dd>
 <dt>   stale
</dt> <dd>      A response is stale if its age has passed its freshness lifetime.
</dd>
 <dt>   semantically transparent
</dt> <dd>      A cache behaves in a "semantically transparent" manner, with
      respect to a particular response, when its use affects neither the
      requesting client nor the origin server, except to improve
      performance. When a cache is semantically transparent, the client
      receives exactly the same response (except for hop-by-hop headers)
      that it would have received had its request been handled directly
      by the origin server.
</dd>
 <dt>   validator
</dt> <dd>      A protocol element (e.g., an entity tag or a Last-Modified time)
      that is used to find out whether a cache entry is an equivalent
      copy of an entity.
</dd>
 <dt>   upstream/downstream
</dt> <dd>      Upstream and downstream describe the flow of a message: all
      messages flow from upstream to downstream.
</dd>
 <dt>   inbound/outbound
</dt> <dd>      Inbound and outbound refer to the request and response paths for
      messages: "inbound" means "traveling toward the origin server",
      and "outbound" means "traveling toward the user agent"
</dd>
</dl>
<h3><a id='sec1.4'>1.4</a> Overall Operation</h3>
<p>
   The HTTP protocol is a request/response protocol. A client sends a
   request to the server in the form of a request method, URI, and
   protocol version, followed by a MIME-like message containing request
   modifiers, client information, and possible body content over a
   connection with a server. The server responds with a status line,
   including the message's protocol version and a success or error code,
   followed by a MIME-like message containing server information, entity
   metainformation, and possible entity-body content. The relationship
   between HTTP and MIME is described in appendix <a rel='xref' href='rfc2616-sec19.html#sec19.4'>19.4</a>.
</p>
<p>
   Most HTTP communication is initiated by a user agent and consists of
   a request to be applied to a resource on some origin server. In the
   simplest case, this may be accomplished via a single connection (v)
   between the user agent (UA) and the origin server (O).
</p>
<pre>          request chain ------------------------>
       UA -------------------v------------------- O
          &lt;----------------------- response chain
</pre>
<p>
   A more complicated situation occurs when one or more intermediaries
   are present in the request/response chain. There are three common
   forms of intermediary: proxy, gateway, and tunnel. A proxy is a
   forwarding agent, receiving requests for a URI in its absolute form,
   rewriting all or part of the message, and forwarding the reformatted
   request toward the server identified by the URI. A gateway is a
   receiving agent, acting as a layer above some other server(s) and, if
   necessary, translating the requests to the underlying server's
   protocol. A tunnel acts as a relay point between two connections
   without changing the messages; tunnels are used when the
   communication needs to pass through an intermediary (such as a
   firewall) even when the intermediary cannot understand the contents
   of the messages.
</p>
<pre>          request chain -------------------------------------->
       UA -----v----- A -----v----- B -----v----- C -----v----- O
          &lt;------------------------------------- response chain
</pre>
<p>
   The figure above shows three intermediaries (A, B, and C) between the
   user agent and origin server. A request or response message that
   travels the whole chain will pass through four separate connections.
   This distinction is important because some HTTP communication options
</p>
<p>
   may apply only to the connection with the nearest, non-tunnel
   neighbor, only to the end-points of the chain, or to all connections
   along the chain. Although the diagram is linear, each participant may
   be engaged in multiple, simultaneous communications. For example, B
   may be receiving requests from many clients other than A, and/or
   forwarding requests to servers other than C, at the same time that it
   is handling A's request.
</p>
<p>
   Any party to the communication which is not acting as a tunnel may
   employ an internal cache for handling requests. The effect of a cache
   is that the request/response chain is shortened if one of the
   participants along the chain has a cached response applicable to that
   request. The following illustrates the resulting chain if B has a
   cached copy of an earlier response from O (via C) for a request which
   has not been cached by UA or A.
</p>
<pre>          request chain ---------->
       UA -----v----- A -----v----- B - - - - - - C - - - - - - O
          &lt;--------- response chain
</pre>
<p>
   Not all responses are usefully cacheable, and some requests may
   contain modifiers which place special requirements on cache behavior.
   HTTP requirements for cache behavior and cacheable responses are
   defined in section 13.
</p>
<p>
   In fact, there are a wide variety of architectures and configurations
   of caches and proxies currently being experimented with or deployed
   across the World Wide Web. These systems include national hierarchies
   of proxy caches to save transoceanic bandwidth, systems that
   broadcast or multicast cache entries, organizations that distribute
   subsets of cached data via CD-ROM, and so on. HTTP systems are used
   in corporate intranets over high-bandwidth links, and for access via
   PDAs with low-power radio links and intermittent connectivity. The
   goal of HTTP/1.1 is to support the wide diversity of configurations
   already deployed while introducing protocol constructs that meet the
   needs of those who build web applications that require high
   reliability and, failing that, at least reliable indications of
   failure.
</p>
<p>
   HTTP communication usually takes place over TCP/IP connections. The
   default port is TCP 80 [19], but other ports can be used. This does
   not preclude HTTP from being implemented on top of any other protocol
   on the Internet, or on other networks. HTTP only presumes a reliable
   transport; any protocol that provides such guarantees can be used;
   the mapping of the HTTP/1.1 request and response structures onto the
   transport data units of the protocol in question is outside the scope
   of this specification.
</p>
<p>
   In HTTP/1.0, most implementations used a new connection for each
   request/response exchange. In HTTP/1.1, a connection may be used for
   one or more request/response exchanges, although connections may be
   closed for a variety of reasons (see section <a rel='xref' href='rfc2616-sec8.html#sec8.1'>8.1</a>).
</p>
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